In the general formula (I), the symbol R represents a methyl or ethyl radical and the symbol Y represents a hydrogen atom or a methylamino or dimethylamino radical or a radical of structure: ##STR3## in which R' is an amino-protecting radical.
As an example, R' may advantageously be selected from trifluoroacetyl, benzyloxycarbonyl, 2-propenyloxycarbonyl, nitrobenzyloxycarbonyl, 9-fluorenylmethyloxycarbonyl or o-nitrobenzyloxyacetyl.